The present invention relates to a method for making balloons for catheters used in medical dilatation procedures.
Balloon catheters are being used extensively in procedures related to the treatment of blood vessels. For example, arterial stenosis is commonly treated by angioplasty procedures which involve inserting balloon catheters into specific arteries. Balloon catheters have also been found useful in procedures involving dilation of body cavities.
The most widely used form of angioplasty makes use of a dilatation catheter which has an inflatable balloon at its distal end. Using fluoroscopy, a physician guides the catheter through the vascular system until the balloon is positioned across the stenoses. The balloon is then inflated by supplying liquid under pressure through an inflation lumen to the balloon. The inflation of the balloon causes stretching of a blood vessel and pressing of the lesion into the blood vessel wall to reestablish acceptable blood flow through the blood vessel.
In order to treat very tight stenoses with small openings, there has been a continuing effort to reduce the profile of the catheter so that the catheter can reach and pass through the small opening of the stenoses. There has also been an effort to reduce the profile of the catheter after an initial use and deflation of the balloon to permit passage of the catheter through additional lesions that are to be treated or to allow entry and retreatment of lesions that reclose after initial treatment.
One factor manipulated to reduce the profile of the dilatation catheter is the wall thickness of the balloon material. Balloons for dilatation balloon catheters have been made from a wide variety of polymeric materials. Typically the balloon wall thicknesses have been on the order of 0.0004 to 0.003 inches for most materials. There have been continuing efforts, however, to develop ever thinner walled balloon materials, while still retaining the necessary distensibility and burst pressure rating, so as to permit lower deflated profiles.
It is possible to make balloons from a variety of materials that are generally of the thermoplastic polymeric type. Such materials may include: polyethylenes and ionomers, ethylene-butylene-styrene block copolymers blended with low molecular weight polystyrene and, optionally, polypropylene, and similar compositions substituting butadiene or isoprene in place of the ethylene and butylene; poly(vinyl chloride); polyurethanes; copolyesters; thermoplastic rubbers; silicone-polycarbonate copolymers; polyamides; and ethylene-vinyl acetate copolymers. Orientable polyesters, especially polyethylene terephthalate (PET), are among the preferred materials for forming catheter balloons.
References illustrating the materials and methods of making catheter balloons include: U.S. Pat. No. 4,413,989 and U.S. Pat. No. 4,456,000 to Schjeldahl et al, U.S. Re 32,983 and Re 33,561 to Levy, and U.S. Pat. No. 4,906,244, U.S. Pat. No. 5,108,415 and U.S. Pat. No. 5,156,612 to Pinchuck et al. The Levy patents, teach that a high tensile strength polyethylene terephthalate balloon can only be formed from a high intrinsic viscosity polymer, specifically, high molecular weight polyethylene terephthalate having a requisite intrinsic viscosity of at least 1.0.
High tensile strengths are important in angioplasty balloons because they allow for the use of high pressure in a balloon having a relatively small wall thickness. High pressure is often needed to treat some forms of stenosis. Small wall thicknesses enable the deflated balloon to remain narrow, making it easier to advance the balloon through the arterial system.
Polyesters possessing a lower intrinsic viscosity are easier to process, and hence balloon manufacturers have desired to use polyesters possessing an intrinsic viscosity below 1.0. However, it was thought that using such material would sacrifice the strength of the balloon. Recently it has been discovered that angioplasty catheter balloons, having a wall strength of greater than 30,000 psi and a burst strength of greater than 300 psi, can be prepared from a PET polymer of an intrinsic viscosity of 0.64-0.8. This, high strength, non-compliant balloon, made from a standard intrinsic viscosity polyester, has been a significant improvement in the art. There remains, however, a need to continue to improve balloon wall strengths while simultaneously reducing their wall thickness.
Prior art PET balloon forming techniques involve blowing or stretching and blowing of the balloon in a segment of extruded PET tubing. It has been recognized that control of moisture in the PET resin, prior to extrusion, is important and prior art techniques have embodied a drying step prior to extrusion of PET tubing from which the balloon is formed by stretch blow molding techniques. However it has not been previously suggested that drying of extruded tubing would provide any benefit properties of the balloons produced from the extruded tubing.
Balloons produced by stretching and blowing a tubular preform or "parison" typically have much thicker waist and cone walls than the wall thickness of their body portions. The thicker cone walls contribute to the overall thickness of the catheter, making tracking, crossing and recrossing of lesions more difficult. Further, thick cones interfere with refolding of the balloon on deflation so that the deflated balloon can only be further inserted or withdrawn with difficulty, occasionally even damaging the blood vessel.
There have been several solutions proposed for reducing the cone or waist thickness of catheter balloons in U.S. Pat. No. 4,906,241, U.S. Pat. No. 4,963,313, and EP 485,903. However, the procedures involved in these references are quite cumbersome and so it is desirable that simplified methods be developed to provide cone and waist walls with reduced thicknesses.